The invention concerns a device for welding thin-walled work pieces such as e.g. food containers made from foil, fibrous web, or foil-fibrous web composites using ultrasound, the device comprising at least two components, i.e. a sonotrode and a counter tool for the sonotrode, wherein one component comprises a working region which faces a working region of the other component, at least during the welding process.
A device of this type is known from DE 100 27 735 A1 (assigned to the assignee of the instant invention), which describes an ultrasound-welding device comprising a sonotrode and an opposing support (also called anvil). The sonotrode and the opposing support each have working edges between which the work piece to be welded is clamped during the welding process.
The conventional device is mainly used to produce containers for food such as e.g. milk, juices, etc. During welding, the maximum temperature is achieved in that region of the work piece to be welded where the amplitude and the mechanical pressure of the ultrasound waves introduced into the work piece reach their maximum. The sonotrode thereby couples the ultrasound energy into the work piece.
It is the object of the present invention to further develop a device of the above-mentioned type such that the welding seams produced thereby are of consistently high quality and density from work piece to work piece.
A device of the above-mentioned type achieves this object in that the working region of one component has at least one projection and the working region of the other component has at least one recess into which the projection can engage during the welding process.
In the inventive device, the sonotrode and the counter tool securely hold the work piece to be welded in a fixed position relative to the sonotrode and the counter tool during the welding process. This prevents the work piece from being displaced relative to the working regions of the sonotrode and/or the counter tools during the welding process, in a simple and inexpensive fashion.
Such a displacement of the work piece tends to occur; in particular, when the containers are filled with liquid, since during filling of the liquid into the containers in the filling device the container outer wall is always moistened with the liquid being filled which makes the surface of the work piece xe2x80x9cslipperyxe2x80x9d. This reduces the friction between the work piece and the working regions of the sonotrode and the counter tool abutting the work piece, and has conventionally impeded secure holding of the work piece to be welded by the sonotrode and the opposing support. If the work piece is displaced during the welding process, the energy is coupled into a larger material region, which can produce a leaking welding seam.
During the welding process, the projection and the recess on the sonotrode or counter tool deform, clamp and thereby securely fix a region of the work piece directly adjacent to the region to be welded. Even for work pieces with smooth and/or slippery surfaces, reproducible welding seam quality is guaranteed. It is thereby clear that the dimensions of the projection and of the recess are matched such that a gap between the wall of the projection and the wall of the recess is produced in which the container wall can be received without being damaged. Additional tensioning and clamping means are not required for holding the work piece. This reduces the price of the inventive device and facilitates cleaning since fewer overall parts are required and no additional undercuts are produced. Since the work piece is safely held relative to the sonotrode and the counter tool, smooth welding lines can also be realized. Additional introduction of contours in the welding region is therefore not required.
Advantageous further developments of the invention are recited in the dependent claims.
In a first preferred embodiment of the inventive device, at least one projection is provided on the counter tool and at least one recess is provided on the sonotrode. This is advantageous for manufacture and produces optimum energy performance of the sonotrode in the region of the work piece to be welded.
A further development proposes that the working region of the counter tool has a cosmetic seam region and a sealing seam region and that the projection is disposed between the cosmetic seam region and sealing seam region. This arrangement provides the finished welded work piece with a particularly attractive visual appearance. At the same time, the region of the work piece, which is held by the sonotrode and the counter tool, is directly adjacent to the actual seam regions such that the welding quality of both seam regions is highly reproducible.
Advantageously, at least one projection is formed as a bridge, which extends along the longitudinal extension of one component, with at least one recess being formed as a groove which extends along the longitudinal extension of the other component and which is complementary to the bridge. The bridge and the groove preferably extend along the entire length of the sonotrode and the counter tool. This guarantees optimum clamping of the work piece between the sonotrode and the counter tool during welding. Alternatively, at least one pin may be provided instead of the bridge and/or at least one hole may be provided instead of the groove.
In a particularly advantageous fashion, the projection is disposed on a part which is separate from the rest of the component and which can be exchanged. This takes into consideration the fact that the projection is worn with time by the numerous clamping processes which such a welding device must perform. The inventive further development permits easy and inexpensive replacement of the projection without having to replace the entire component. This reliably prevents damage during the clamping process.
It may be advantageous to production when the projection is integrated in the component as a single piece. The stability of the projection is also particularly good in this further development of the inventive device.
In one embodiment of the inventive device, the projecting end of the projection and/or the edge of the recess has an insertion bevel. This insertion bevel(s) prevent(s) damage to the work piece to be processed when a region adjacent to the region to be welded is clamped between the sonotrode and the counter tool.
The invention also concerns a method for welding thin-walled work pieces through ultrasound such as e.g. food containers of foil, fibrous web or of foil-fibrous web composites, wherein the work piece is disposed between the working regions of two components, namely a sonotrode and a counter tool, wherein the working regions face each other at least during the welding process.
To guarantee the quality of the welding seam between the two work pieces which is produced by this method, i.e. such that it is reproducible, the invention proposes loading and deforming a region of the work piece during the welding process using a projection which is provided on one component and which engages in a recess provided on the opposite component.